Pressure control

ABSTRACT

A method and apparatus is disclosed for determining pressure provided by a pump of a topical negative pressure (TNP) system. The method includes the steps of determining a value associated with a measured pressure, determining a difference between the measured pressure value and a predetermined desired value and increasing or decreasing a speed of pumping of the pump responsive the to difference.

The present invention relates to apparatus and a method for theapplication of topical negative pressure (TNP) therapy to wounds. Inparticular, but not exclusively, the present invention relates to amethod and apparatus for providing a desired pressure at a wound site byincreasing or decreasing pump speed.

There is much prior art available relating to the provision of apparatusand methods of use thereof for the application of TNP therapy to woundstogether with other therapeutic processes intended to enhance theeffects of the TNP therapy. Examples of such prior art include thoselisted and briefly described below.

TNP therapy assists in the closure and healing of wounds by reducingtissue oedema; encouraging blood flow and granulation of tissue;removing excess exudates and may reduce bacterial load and thus,infection to the wound. Furthermore, TNP therapy permits less outsidedisturbance of the wound and promotes more rapid healing.

In our co-pending International patent application, WO 2004/1037334,apparatus, a wound dressing and a method for aspirating, irrigating andcleansing wounds are described. In very general terms, this inventiondescribes the treatment of a wound by the application of topicalnegative pressure (TNP) therapy for aspirating the wound together withthe further provision of additional fluid for irrigating and/orcleansing the wound, which fluid, comprising both wound exudates andirrigation fluid, is then drawn off by the aspiration means andcirculated through means for separating the beneficial materials thereinfrom deleterious materials. The materials which are beneficial to woundhealing are recirculated through the wound dressing and those materialsdeleterious to wound healing are discarded to a waste collection bag orvessel.

In our co-pending International patent application, WO 2005/04670,apparatus, a wound dressing and a method for cleansing a wound usingaspiration, irrigation and cleansing wounds are described. Again, invery general terms, the invention described in this document utilisessimilar apparatus to that in WO 2004/037334 with regard to theaspiration, irrigation and cleansing of the wound, however, it furtherincludes the important additional step of providing heating means tocontrol the temperature of that beneficial material being returned tothe wound site/dressing so that it is at an optimum temperature, forexample, to have the most efficacious therapeutic effect on the wound.In our co-pending International patent application, WO 2005/105180,apparatus and a method for the aspiration, irrigation and/or cleansingof wounds are described. Again, in very general terms, this documentdescribes similar apparatus to the two previously mentioned documentshereinabove but with the additional step of providing means for thesupply and application of physiologically active agents to the woundsite/dressing to promote wound healing.

The content of the above references is included herein by reference.

However, the above apparatus and methods are generally only applicableto a patient when hospitalised as the apparatus is complex, needingpeople having specialist knowledge in how to operate and maintain theapparatus, and also relatively heavy and bulky, not being adapted foreasy mobility outside of a hospital environment by a patient, forexample.

Some patients having relatively less severe wounds which do not requirecontinuous hospitalisation, for example, but whom nevertheless wouldbenefit from the prolonged application of TNP therapy, could be treatedat home or at work subject to the availability of an easily portable andmaintainable TNP therapy apparatus.

GB-A-2 307 180 describes a portable TNP therapy unit which may becarried by a patient clipped to belt or harness. A negative pressure isapplied at a wound site but that pressure is prone to fluctuation whichmay have a detrimental effect to healing.

In known portable devices which carry a suction pump it is known that anegative pressure provided by the pump can fluctuate over time or due tocertain environmental conditions. As a result a healing effect may belessened or other adverse effects may be caused. Also in certain knowntechniques there is little or no ability to control pressure provided bya pump by a user. Still furthermore certain devices provide a limitedamount of pressure control but that control is prone to sudden changewhich may cause jitter and noise which may concern a user. It will alsobe appreciated that a lack of smooth speed responses can reduceoperational lifetimes as the pump rarely experiences full duty cycles.Still furthermore fluctuations in pressure can result in pain to a user.

It is an aim of the present invention to at least partly mitigate theabove-mentioned problems.

It is an aim of embodiments of the present invention to provide a methodand apparatus for controlling pressure provided by a pump of a topicalnegative pressure (TNP) system.

It is an aim of embodiments of the present invention to provide a methodand apparatus for controlling pressure so that jitter and noise causedby sudden changes in pump speed are reduced or avoided altogether.

It is an aim of embodiments of the present invention to provide a TNPsystem which has a pressure which can be set and modified convenientlyby a user.

According to a first aspect of the present invention there is provided amethod of determining pressure provided by a pump of a topical negativepressure (TNP) system, the method comprising the steps of:

-   -   determining a value associated with a measured pressure;    -   determining a difference between the measured pressure value and        a predetermined desired value; and    -   increasing or decreasing a speed of pumping of the pump        responsive to the difference.

The invention is comprised in part of an overall apparatus for theprovision of TNP therapy to a patient in almost any environment. Theapparatus is lightweight, may be mains or battery powered by arechargeable battery pack contained within a device (henceforth, theterm “device” is used to connote a unit which may contain all of thecontrol, power supply, power supply recharging, electronic indicatormeans and means for initiating and sustaining aspiration functions to awound and any further necessary functions of a similar nature). Whenoutside the home, for example, the apparatus may provide for an extendedperiod of operation on battery power and in the home, for example, thedevice may be connected to the mains by a charger unit whilst stillbeing used and operated by the patient.

The overall apparatus of which the present invention is a partcomprises: a dressing covering the wound and sealing at least an openend of an aspiration conduit to a cavity formed over the wound by thedressing; an aspiration tube comprising at least one lumen therethroughleading from the wound dressing to a waste material canister forcollecting and holding wound exudates/waste material prior to disposal;and, a power, control and aspiration initiating and sustaining deviceassociated with the waste canister.

The dressing covering the wound may be any type of dressing normallyemployed with TNP therapy and, in very general terms, may comprise, forexample, a semi-permeable, flexible, self-adhesive drape material, as isknown in the dressings art, to cover the wound and seal with surroundingsound tissue to create a sealed cavity or void over the wound. There mayaptly be a porous barrier and support member in the cavity between thewound bed and the covering material to enable an even vacuumdistribution to be achieved over the area of the wound. The porousbarrier and support member being, for example, a gauze, a foam, aninflatable bag or known wound contact type material resistant tocrushing under the levels of vacuum created and which permits transferof wound exudates across the wound area to the aspiration conduit sealedto the flexible cover drape over the wound.

The aspiration conduit may be a plain flexible tube, for example, havinga single lumen therethrough and made from a plastics material compatiblewith raw tissue, for example. However, the aspiration conduit may have aplurality of lumens therethrough to achieve specific objectives relatingto the invention. A portion of the tube sited within the sealed cavityover the wound may have a structure to enable continued aspiration andevacuation of wound exudates without becoming constricted or blockedeven at the higher levels of the negative pressure range envisaged.

It is envisaged that the negative pressure range for the apparatusembodying the present invention may be between about −50 mmHg and −200mmHg (note that these pressures are relative to normal ambientatmospheric pressure thus, −200 mmHg would be about 560 mmHg inpractical terms). Aptly, the pressure range may be between about −75mmHg and −150 mmHg. Alternatively a pressure range of upto −75 mmHg,upto −80 mmHg or over −80 mmHg can be used. Also aptly a pressure rangeof below −75 mmHg could be used. Alternatively a pressure range of over−100 mmHg could be used or over −150 mmHg.

The aspiration conduit at its distal end remote from the dressing may beattached to the waste canister at an inlet port or connector. The devicecontaining the means for initiating and sustaining aspiration of thewound/dressing may be situated between the dressing and waste canister,however, in a preferred embodiment of the apparatus embodying thepresent invention, the device may aspirate the wound/dressing via thecanister thus, the waste canister may preferably be sited between thewound/dressing and device.

The aspiration conduit at the waste material canister end may preferablybe bonded to the waste canister to prevent inadvertent detachment whenbeing caught on an obstruction, for example.

The canister may be a plastics material moulding or a composite unitcomprising a plurality of separate mouldings. The canister may aptly betranslucent or transparent in order to visually determine the extent offilling with exudates. However, the canister and device may in someembodiments provide automatic warning of imminent canister fullcondition and may also provide means for cessation of aspiration whenthe canister reaches the full condition.

The canister may be provided with filters to prevent the exhaust ofliquids and odours therefrom and also to prevent the expulsion ofbacteria into the atmosphere. Such filters may comprise a plurality offilters in series. Examples of suitable filters may comprise hydrophobicfilters of 0.2 μm pore size, for example, in respect of sealing thecanister against bacteria expulsion and 1 μm against liquid expulsion.

Aptly, the filters may be sited at an upper portion of the wastecanister in normal use, that is when the apparatus is being used orcarried by a patient the filters are in an upper position and separatedfrom the exudate liquid in the waste canister by gravity. Furthermore,such an orientation keeps the waste canister outlet or exhaust exit portremote from the exudate surface.

Aptly the waste canister may be filled with an absorbent gel such asISOLYSEL (trade mark), for example, as an added safeguard againstleakage of the canister when full and being changed and disposed of.Added advantages of a gel matrix within the exudate storing volume ofthe waste canister are that it prevents excessive movement, such asslopping, of the liquid, minimises bacterial growth and minimisesodours.

The waste canister may also be provided with suitable means to preventleakage thereof both when detached from the device unit and also whenthe aspiration conduit is detached from the wound site/dressing.

The canister may have suitable means to prevent emptying by a user(without tools or damage to the canister) such that a full or otherwiseend-of-life canister may only be disposed of with waste fluid stillcontained.

The device and waste canister may have mutually complementary means forconnecting a device unit to a waste canister whereby the aspirationmeans in the device unit automatically connects to an evacuation port onthe waste canister such that there is a continuous aspiration path fromthe wound site/dressing to an exhaust port on the device.

Aptly, the exhaust port from the fluid path through the apparatus isprovided with filter means to prevent offensive odours from beingejected into the atmosphere.

In general terms the device unit comprises an aspirant pump; means formonitoring pressure applied by the aspirant pump; a flowmeter to monitorfluid flow through the aspirant pump; a control system which controlsthe aspirant pump in response to signals from sensors such as thepressure monitoring means and the flowmeter, for example, and whichcontrol system also controls a power management system with regard to anon-board battery pack and the charging thereof and lastly a userinterface system whereby various functions of the device such aspressure level set point, for example, may be adjusted (includingstopping and starting of the apparatus) by a user. The device unit maycontain all of the above features within a single unified casing.

In view of the fact that the device unit contains the majority of theintrinsic equipment cost therein ideally it will also be able to surviveimpact, tolerate cleaning in order to be reusable by other patients.

In terms of pressure capability the aspiration means may be able toapply a maximum pressure drop of at least −200 mmHg to a woundsite/dressing. The apparatus is capable of maintaining a predeterminednegative pressure even under conditions where there is a small leak ofair into the system and a high exudate flow.

The pressure control system may prevent the minimum pressure achievedfrom exceeding −200 mmHg so as not to cause undue patient discomfort.The pressure required may be set by the user at a number of discreetlevels such as −50, −75, −100, −125, −150, −175 mmHg, for example,depending upon the needs of the wound in question and the advice of aclinician. The control system may also advantageously be able tomaintain the set pressure within a tolerance band of +/−10 mmHg of theset point for 95% of the time the apparatus is operating given thatleakage and exudation rates are within expected or normal levels.

Aptly, the control system may trigger alarm means such as a flashinglight, buzzer or any other suitable means when various abnormalconditions apply such as, for example: pressure outside set value by alarge amount due to a gross leak of air into system; duty on theaspiration pump too high due to a relatively smaller leakage of air intothe system; pressure differential between wound site and pump is toohigh due, for example, to a blockage or waste canister full.

The apparatus of the present invention may be provided with a carry caseand suitable support means such as a shoulder strap or harness, forexample. The carry case may be adapted to conform to the shape of theapparatus comprised in the joined together device and waste canister. Inparticular, the carry case may be provided with a bottom opening flap topermit the waste canister to be changed without complete removal of theapparatus form the carry case.

The carry case may be provided with an aperture covered by adisplaceable flap to enable user access to a keypad for varying thetherapy applied by the apparatus.

According to a second aspect of the present invention, there is providedapparatus that controls pressure provided by a pump of a topicalnegative pressure TNP system, comprising:

-   -   a pressure sensor that measures pressure at an inlet to the        pump;    -   a user interface via which a user can indicate a predetermined        desired value; and    -   a processing unit that compares a value, associated with the        measured pressure, with said predetermined value and increases        or decreases a speed of pumping of the pump responsive to the        compared result.

In order that the present invention may be more fully understood,examples will now be described by way of illustration only withreference to the accompanying drawings, of which:

FIG. 1 shows a generalised schematic block diagram showing a generalview of an apparatus and the constituent apparatus features thereof;

FIG. 2 shows a similar generalised schematic block diagram to FIG. 1 andshowing fluid paths therein;

FIG. 3 shows a generalised schematic block diagram similar to FIG. 1 butof a device unit only and showing power paths for the various powerconsuming/producing features of the apparatus;

FIG. 4 shows a similar generalised schematic block diagram to FIG. 3 ofthe device unit and showing control system data paths for controllingthe various functions and components of the apparatus;

FIG. 5 shows a perspective view of an apparatus;

FIG. 6 shows a perspective view of an assembled device unit of theapparatus of FIG. 5;

FIG. 7 shows an exploded view of the device unit of FIG. 6;

FIG. 8 shows a partially sectioned side elevation view through theinterface between a waste canister and device unit of the apparatus;

FIG. 9 shows a cross section through a waste canister of the apparatusof FIGS. 5 to 8;

FIG. 10 illustrates how pressure control can be achieved; and

FIG. 11 illustrates how pressure generated by a pump can be controlled.

Referring now to FIGS. 1 to 4 of the drawings and where the same orsimilar features are denoted by common reference numerals.

FIG. 1 shows a generalised schematic view of an apparatus 10 of aportable topical negative pressure (TNP) system. It will be understoodthat embodiments of the present invention are generally applicable touse in such a TNP system. Briefly, negative pressure wound therapyassists in the closure and healing of many forms of “hard to heal”wounds by reducing tissue oedema; encouraging blood flow and granulartissue formation; removing excess exudate and may reduce bacterial load(and, therefore, infection). In addition the therapy allows for lessdisturbance of a wound leading to more rapid healing. The TNP system isdetailed further hereinafter but in summary includes a portable bodyincluding a canister and a device with the device capable of providingan extended period of continuous therapy within at least a one year lifespan. The system is connected to a patient via a length of tubing withan end of the tubing operably secured to a wound dressing on thepatient.

More particularly, as shown in FIG. 1, the apparatus comprises anaspiration conduit 12 operably and an outer surface thereof at one endsealingly attached to a dressing 14. The dressing 14 will not be furtherdescribed here other than to say that it is formed in a known mannerfrom well know materials to those skilled in the dressings art to createa sealed cavity over and around a wound to be treated by TNP therapywith the apparatus of the present invention. The aspiration conduit hasan in-line connector 16 comprising connector portions 18, 20intermediate its length between the dressing 14 and a waste canister 22.The aspiration conduit between the connector portion 20 and the canister22 is denoted by a different reference numeral 24 although the fluidpath through conduit portions 12 and 24 to the waste canister iscontinuous. The connector portions 18, 20 join conduit portions 12, 24in a leak-free but disconnectable manner. The waste canister 22 isprovided with filters 26 which prevent the escape via an exit port 28 ofliquid and bacteria from the waste canister. The filters may comprise a1 μm hydrophobic liquid filter and a 0.2 μm bacteria filter such thatall liquid and bacteria is confined to an interior waste collectingvolume of the waste canister 22. The exit port 28 of the waste canister22 mates with an entry/suction port 30 of a device unit 32 by means ofmutually sealing connector portions 34, 36 which engage and sealtogether automatically when the waste canister 22 is attached to thedevice unit 32, the waste canister 22 and device unit 32 being heldtogether by catch assemblies 38, 40. The device unit 32 comprises anaspirant pump 44, an aspirant pressure monitor 46 and an aspirantflowmeter 48 operably connected together. The aspiration path takes theaspirated fluid which in the case of fluid on the exit side of exit port28 is gaseous through a silencer system 50 and a final filter 52 havingan activated charcoal matrix which ensures that no odours escape withthe gas exhausted from the device 32 via an exhaust port 54. The filter52 material also serves as noise reducing material to enhance the effectof the silencer system 50. The device 32 also contains a battery pack 56to power the apparatus which battery pack also powers the control system60 which controls a user interface system 62 controlled via a keypad(not shown) and the aspiration pump 44 via signals from sensors 46, 48.A power management system 66 is also provided which controls power fromthe battery pack 56, the recharging thereof and the power requirementsof the aspirant pump 44 and other electrically operated components. Anelectrical connector 68 is provided to receive a power input jack 70from a SELV power supply 72 connected to a mains supply 74 when the userof the apparatus or the apparatus itself is adjacent a convenient mainspower socket.

FIG. 2 shows a similar schematic representation to FIG. 1 but shows thefluid paths in more detail. The wound exudate is aspirated from thewound site/dressing 14 via the conduit 12, the two connector portions18, 20 and the conduit 24 into the waste canister 22. The waste canister22 comprises a relatively large volume 80 in the region of 500 ml intowhich exudate from the wound is drawn by the aspiration system at anentry port 82. The fluid 84 drawn into the canister volume 80 is amixture of both air drawn into the dressing 14 via the semi-permeableadhesive sealing drape (not shown) and liquid 86 in the form of woundexudates. The volume 80 within the canister is also at a loweredpressure and the gaseous element 88 of the aspirated fluids is exhaustedfrom the canister volume 80 via the filters 26 and the waste canisterexhaust exit port 28 as bacteria-free gas. From the exit port 28 of thewaste canister to the final exhaust port 54 the fluid is gaseous only.

FIG. 3 shows a schematic diagram showing only the device portion of theapparatus and the power paths in the device of the apparatus embodyingthe present invention. Power is provided mainly by the battery pack 56when the user is outside their home or workplace, for example, however,power may also be provided by an external mains 74 supplied chargingunit 72 which when connected to the device 32 by the socket 68 iscapable of both operating the device and recharging the battery pack 56simultaneously. The power management system 66 is included so as to beable to control power of the TNP system. The TNP system is arechargeable, battery powered system but is capable of being rundirectly from mains electricity as will be described hereinafter morefully with respect to the further figures. If disconnected from themains the battery has enough stored charge for approximately 8 hours ofuse in normal conditions. It will be appreciated that batteries havingother associated life times between recharge can be utilised. Forexample batteries providing less than 8 hours or greater than 8 hourscan be used. When connected to the mains the device will run off themains power and will simultaneously recharge the battery if depletedfrom portable use. The exact rate of battery recharge will depend on theload on the TNP system. For example, if the wound is very large or thereis a significant leak, battery recharge will take longer than if thewound is small and well sealed.

FIG. 4 shows the device 32 part of the apparatus embodying the presentinvention and the data paths employed in the control system for controlof the aspirant pump and other features of the apparatus. A key purposeof the TNP system is to apply negative pressure wound therapy. This isaccomplished via the pressure control system which includes the pump anda pump control system. The pump applies negative pressure; the pressurecontrol system gives feedback on the pressure at the pump head to thecontrol system; the pump control varies the pump speed based on thedifference between the target pressure and the actual pressure at thepump head. In order to improve accuracy of pump speed and hence providesmoother and more accurate application of the negative pressure at awound site, the pump is controlled by an auxiliary control system. Thepump is from time to time allowed to “free-wheel” during its duty cycleby turning off the voltage applied to it. The spinning motor causes a“back electro-motive force” or BEMF to be generated. This BEMF can bemonitored and can be used to provide an accurate measure of pump speed.The speed can thus be adjusted more accurately than can prior art pumpsystems.

According to embodiments of the present invention, actual pressure at awound site is not measured but the difference between a measuredpressure (at the pump) and the wound pressure is minimised by the use oflarge filters and large bore tubes wherever practical. If the pressurecontrol measures that the pressure at the pump head is greater than atarget pressure (closer to atmospheric pressure) for a period of time,the device sends an alarm and displays a message alerting the user to apotential problem such as a leak.

In addition to pressure control a separate flow control system can beprovided. A flow meter may be positioned after the pump and is used todetect when a canister is full or the tube has become blocked. If theflow falls below a certain threshold, the device sounds an alarm anddisplays a message alerting a user to the potential blockage or fullcanister.

Referring now to FIGS. 5 to 9 which show various views and crosssections of a preferred embodiment of apparatus 200 embodying thepresent invention. The preferred embodiment is of generally oval shapein plan and comprises a device unit 202 and a waste canister 204connected together by catch arrangements 206. The device unit 202 has aliquid crystal display (LCD) 208, which gives text based feedback on thewound therapy being applied, and a membrane keypad 210, the LCD beingvisible through the membrane of the keypad to enable a user to adjust orset the therapy to be applied to the wound (not shown). The device has alower, generally transverse face 212 in the centre of which is a spigot214 which forms the suction/entry port 216 to which the aspiration means(to be described below) are connected within the device unit. The loweredge of the device unit is provided with a rebated peripheral malemating face 218 which engages with a co-operating peripheral femaleformation 220 on an upper edge of the waste canister 204 (see FIGS. 8and 9). On each side of the device 202, clips 222 hinged to the canister204 have an engaging finger (not shown) which co-operates withformations in recesses 226 in the body of the device unit. From FIG. 7it may be seen that the casing 230 of the device unit is of largely“clamshell” construction comprising front and back mouldings 232, 234,respectively and left-hand and right-hand side inserts 236, 238. Insidethe casing 230 is a central chassis 240 which is fastened to an internalmoulded structural member 242 and which chassis acts as a mounting forthe electrical circuitry and components and also retains the batterypack 246 and aspiration pump unit 248. Various tubing items 250, 252,254 connect the pump unit 248 and suction/entry port 216 to a finalgaseous exhaust via a filter 290. FIG. 8 shows a partially sectionedside elevation of the apparatus 200, the partial section being aroundthe junction between the device unit 202 and the waste canister 204, across section of which is shown at FIG. 9. Theses views show the rebatededge 218 of the male formation on the device unit co-operating with thefemale portion 220 defined by an upstanding flange 260 around the topface 262 of the waste canister 204. When the waste canister is joined tothe device unit, the spigot 214 which has an “O” ring seal 264therearound sealingly engages with a cylindrical tube portion 266 formedaround an exhaust/exit port 268 in the waste canister. The spigot 214 ofthe device is not rigidly fixed to the device casing but is allowed to“float” or move in its location features in the casing to permit thespigot 214 and seal 264 to move to form the best seal with the bore ofthe cylindrical tube portion 266 on connection of the waste canister tothe device unit. The waste canister 204 in FIG. 9 is shown in an uprightorientation much as it would be when worn by a user. Thus, any exudate270 would be in the bottom of the internal volume of waste receptacleportion 272. An aspiration conduit 274 is permanently affixed to anentry port spigot 278 defining an entry port 280 to receive fluidaspirated from a wound (not shown) via the conduit 274. Filter members282 comprising a 0.2 μm filter and 284 comprising a 1 μm filter arelocated by a filter retainer moulding 286 adjacent a top closure memberor bulkhead 288 the filter members preventing any liquid or bacteriafrom being drawn out of the exhaust exit port 268 into the pump andaspiration path through to an exhaust and filter unit 290 which isconnected to a casing outlet moulding at 291 via an exhaust tube (notshown) in casing side piece 236. The side pieces 236, 238 are providedwith recesses 292 having support pins 294 therein to locate a carryingstrap (not shown) for use by the patient. The side pieces 230 andcanister 204 are also provided with features which prevent the canisterand device from exhibiting a mutual “wobble” when connected together.Ribs (not shown) extending between the canister top closure member 288and the inner face 300 of the upstanding flange 260 locate in grooves302 in the device sidewalls when canister and device are connected. Thecasing 230 also houses all of the electrical equipment and control andpower management features, the functioning of which was describedbriefly with respect to FIGS. 3 and 4 hereinabove. The side piece 238 isprovided with a socket member 298 to receive a charging jack from anexternal mains powered battery charger (both not shown).

FIGS. 10 and 11 illustrate how the pressure provided by a pump of theTNP system can be set, monitored and controlled according to anembodiment of the present invention. As illustrated in FIG. 10 apressure sensor such as a pressure transducer is utilised to measureactual pressure at a pump inlet, which during use will be located at orclose to a wound site. It will be appreciated that according toembodiments of the present invention the pressure sensor may be locatedat some predetermined location along the tube connecting the device unitto the wound site.

Pumping pressure is controlled by an initial pump speed setting systemwhich measures pressure and sets a desired pump speed responsive to themeasured pressure and a predetermined pressure set by a user, and afurther control loop system in which actual pump speed is monitored andcompared with the determined pump speed. Pumping is actually controlledresponsive to the comparison.

As illustrated in FIG. 10 the pressure determined by the sensor isconverted into a digital value in an analogue digital converter 1000 andthe value scaled to thereby filter pressure reading before being fedinto the control loop to thereby minimise the effect of noise in thereading thereby reducing jitter. This also helps minimise false alarmsdue to over or under pressure situations.

Pump speed control is achieved by implementing a control loop inhardware or software. The measured scaled pressure provides an input1002 into the pressure controller whilst a further input 1003 isprovided by a user entering a desired pressure via a user interface. Thepressure controller 1004 takes the pressure set point and the actualmeasure of pressure as inputs to deliver a new desired pump speed as itsoutput Vset. The measured pressure values from the pressure transducerare averaged over a certain number of previous readings before feeding avalue to the control loop. This minimises jitter and noise and serves asa first dampener of pump response.

The control sequence used for controlling pump response is given below:

Defines >>> Constants for control loop: kp, ki, t Bounds for output :Vmax, Vmin Inputs >>> Current pressure value: pv, Set point value: sp   Calculate difference: e = sp − pv    P = kp * e    I = I + ki * e * t   Verify I is between the Vmin and V max bounds    V = P + I    VerifyV is between the Vmin and V max bounds

Thus the difference between the measured pressure and a desired pressureis calculated and then scaled using experimentally predeterminedconstants to yield the output value of pump speed Vset. The constantsare optimised for best pump response and to minimise pressure overshootor undershoot. The scaling further dampens the effect of the currentpressure difference by taking into account a certain number of previouspressure differences. The control loop is provided to allow only acertain maximum step change in pressure at a time by bounding the outputpump speed value within predetermined sensible limits. Thus a suddenchange in measured pressure (due to any reason for example the userchanging position) or a change in the pressure set point is fed back tothe pump drive circuitry incrementally in small steps rather than as adramatic change.

This mechanism of pump speed control thus results in a better reactionto rapid changes in pressure as the pump does not instantly ‘overreact’.Since the pump does not have a drastic reaction to pressure changes theoverall ‘perceived’ noise levels are lower. Gradual adjustment of pumpspeed also results in lower pump wear and tear which enhances deviceperformance and longevity. Furthermore averaging the pressure transducerreadings before feeding them to the control loop reduces the likelihoodof false alarms with respect to over or under pressure situations.

FIG. 11 illustrates how accurate speed control of a suction pump on theTNP device allows fine control of a negative pressure applied at a woundsite and which thus helps reduce noise during device operation andminimises discomfort to the user. The system provides a control loopthat periodically turns off power to a pump motor and records anelectromotive force (EMF) generated by a freewheeling element such as arotor of the pump. The measured EMF is used to calculate the actual pumpspeed and drive signals supplied to the pump can thus be modified toaccurately achieve a desired speed.

As illustrated in FIG. 11 a control loop 1100 uses the desired andactual pump speeds at a given instant to accurately determine the drivevoltage that needs to be applied to the pump in order to accuratelyachieve final desired speed and thus pressure. The control loop operatesby calculating the difference between the desired speed Vset and thecurrent speed Vcur. The pump controller 1101 scales the difference andoptionally accumulates the scaled differences from a certain number ofprevious iterations. The control loop 1101 outputs a value Vfinal forthe pump drive voltage that leads to the pump achieving its finaldesired speed. The scaling constants for the control are determinedexperimentally prior to operation to ensure acceptable performance ofthe device (ie. ability to maintain set pressure at specified wound leakrates). The scaling constants can be calculated in various ways howeveraptly on a startup conditions to provide a predetermined pressure can beapplied. A measured actual pressure will indicate operational parametersindicative of pressure change, leaks, wound size and volumes in thewaste canister. Scaling constants are then set responsive to these.

The pump control system is responsible for maintaining the pump speedwhich in turn drives the pressure generated at the wound. The motorspeed is controlled by varying a pulse width modulation (PWM) input. Theduty cycle of the PWM generator 1102 is controlled responsive to thedrive voltage signal Vfinal and the output of the PWM generator isutilised to drive the pump 1103.

The actual speed of the pump is obtained by measuring the terminalvoltage across the pump with the current at zero. This is achieved byintermittently turning the pump power off by controlling the PWMgenerator output. Subsequent to turn off a short period is allowed towait for the EMF of the freewheeling pump to settle during a certainpredetermined time period and thereafter the steady value of the EMF issampled. This EMF is a direct measure of pump shaft speed. The EMFgenerated is converted into a digital signal via an analogue digitalconverter 1104 and then scaled with a scaling unit 1105. The EMFsampling rate is varied according to pump speed to counter the aliasingand to minimise the effect on pump speed. The EMF sampling rate may bereduced at higher pump speed since the inertia of the pump maintains amore constant motion at high pump speeds.

Operation of the control utilised can be summarised by the followingcontrol sequence:

Pump_Speed_Controller    Turn pump_enable and PWM off    Turnpump_enable on after current drops    Allow EMF to settle    Sample EMFand estimate current speed (Vcur)    new_PWM = PI (Vset, Vcur)    Enablepump PWM    New pump duty cycle = new_PWM End Motor_Controller PI (Vset,Vcur)    Defines >> constants kp, ki, t    Inputs >> current motor speedVcur Desired motor speed Vset    Calculate difference: e = Vset − Vcur   P = kp * e    I = I + ki * e * t End PI

Accurate pump control results in overall lower noise levels duringdevice operation. Specifically abrupt changes in noise are avoidedbecause the pump speed is adjusted frequently and in small steps.Maintaining accurate control of pump speeds can extend pump and batterylife. Moreover a steady pump delivers a steady negative pressure therebyminimising patient discomfort.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

1. A method for determining pressure provided by a pump of a topicalnegative pressure (TNP) system, the method comprising the steps of:determining a value associated with a measured pressure by averaging themeasured pressure with at least one previous reading of the pressure;determining a difference between the measured pressure value and apredetermined desired value; and increasing or decreasing a speed ofpumping of the pump responsive to the difference.
 2. The method asclaimed in claim 1 wherein said step of determining said value comprisesthe steps of: measuring pressure at a pressure sensor located proximateto a pump inlet; and providing said a value by averaging the measuredpressure with previous readings of the pressure taken for apredetermined period.
 3. The method as claimed in claim 1, furthercomprising the steps of: responsive to a detected sudden change inpressure provided by the pump, repeatedly reducing or increasing apumping speed of the pump to return detected pressure to a pressurecorresponding to the predetermined value.
 4. The method as claimed inclaim 1, further comprising the steps of: setting said predetermineddesired value via a user interface of the TNP system.
 5. The method asclaimed in claim 1, further comprising the steps of: scaling a valuedetermined during said step of determining a difference according to atleast one previously determined constant value and increasing ordecreasing the speed of pumping responsive to said scaled value.
 6. Themethod as claimed in claim 1, further comprising the steps of: providinga respective maximum and minimum pump speed threshold value; andproviding a signal to increase or decrease the speed of pumping onlywithin speeds corresponding to the respective threshold values. 7.Apparatus for controlling pressure provided by a pump of a topicalnegative pressure (TNP) system, comprising: a pressure sensor configuredto measure pressure at an inlet to the pump; a user interface via whicha user can indicate a predetermined desired value; and a processing unitconfigured to compare a value associated with the measured pressure withsaid predetermined value, said processing unit configured to transmit asignal to increase or decrease pumping speed of the pump responsive tothe compared result, the value comprising an average of the measuredpressure and at least one previous reading of the pressure.
 8. Theapparatus as claimed in claim 7, further comprising: the processing unitis further configured to scale the compared result according to at leastone predetermined constant.
 9. The apparatus as claimed in claim 7wherein said processing unit is further configured to increase ordecrease the pumping speed responsive to a pump speed signal, said pumpspeed signal being maintained between respective maximum and minimumpump speed threshold values.
 10. (canceled)
 11. (canceled) 12.(canceled)
 13. The method of claim 1, wherein the predetermined desiredvalue comprises a value between about −50 mmHG and about −200 mmHG. 14.The method of claim 1, wherein the speed of pumping of the pump isincreased or decreased incrementally.
 15. The apparatus of claim 7,wherein the predetermined desired value comprises a value between about−50 mmHG and about −200 mmHG.
 16. The apparatus according to claim 7,wherein the apparatus is part of a TNP system.
 17. The apparatus asclaimed in claim 7, wherein said processing unit is further configuredto increase or decrease the speed of pumping incrementally.